(a) Field of the Invention
The present invention relates to a digital communication method and a system thereof, and specifically to an apparatus and method for a statistical multiplexing of channels based on a multidimensional orthogonal resource hopping method in case where each channel has a variable transmission rate less than a basic transmission rate R in wire/wireless communication systems using a plurality of low-activity communication channels mutually synchronized through a single medium.
More specifically, the present invention relates to a multiplexing apparatus and method in a system composed of a primary station and a plurality of secondary stations mutually synchronized, the primary station identifying a channel to each secondary station using a multidimensional orthogonal resource hopping pattern, the multidimensional orthogonal resource hopping pattern corresponding to the secondary station including an intentional(non-random) hopping pattern allocated by the primary station during a call set up or a pseudo-random hopping pattern unique to the secondary station. The coordinates of the multidimensional orthogonal resources in hopping patterns of a different channel can be identical(matched) (this phenomenon will be referred to as a “multidimensional hopping pattern collision”). In this case, whether or not the channels are matched is determined from the transmit data symbols for all transmit channels of the primary station related to the multidimensional hopping pattern collision. If a data symbol having at least one unmatched channel is transmitted, the corresponding data symbol interval is turned off and the transmission power of all channels off in data symbol transmission may be increased as much as a predetermined amount for a predetermined time as defined by the protocols so as to compensate for a loss of the average bit energy of missing data of all the related channels.
In this description, the primary and secondary stations correspond to a base station and mobile stations, respectively, in the existing systems. The primary station is in communication with multiple secondary stations. The present invention relates to a statistical multiplexing method applicable in a synchronized channel group maintaining orthogonality in the direction from the primary station to the secondary stations.
(b) Description of the Related Art
The present invention can be embodied independently in each channel group for a system maintaining orthogonality only in each channel group, e.g., a quasi-orthogonal code (QOC) used in the cdma2000 system that is a candidate technology of the next generation mobile communication system under standardization, i.e., IMT-2000, or a multi-scrambling code (MSC) to be used in the WCDMA system. With the channels of a primary station classified into channel groups having a same transmitter antenna beam as in a sectorizing or smart antenna system, the present invention can also be embodied independently in each channel
For expediency in explaining which part of the prior art is modified in the embodiment of the multiplexing system of the present invention, the following description will be given on the basis of the IS-95 (cdmaOne) system that is a conventional mobile communication system now in commercial use.
In the digital/analog FDM (Frequency Division Multiplexing) communication system according to prior art, a primary station allocates available FA (Frequency Allocation) to a secondary station irrespective of the channel activity during a call set up, and the secondary station returns the FA to the primary station for another secondary station after termination of the call.
In the TDM (Time Division Multiplexing) communication system according to prior art, a primary station allocates one of available time slots in one FA to a secondary station irrespective of the channel activity during a call set up, and the secondary station returns the time slot to the primary station for another secondary station after termination of the call.
In the FHM (Frequency Hopping Multiplexing) communication system according to prior art, a primary station is in communication with a secondary station using a negotiated frequency hopping pattern irrespective of the channel activity during a call set up, and determines whether to allocate a new channel according to the number of allocated channels. But the FHM system has no control function of the present invention for not sending symbols of the related channel in order to reduce possible errors at the channel decoder of the receiver in the case of a hopping pattern collision.
In the OCDM (Orthogonal Code Division Multiplexing) communication system according to prior art, a primary station allocates an available orthogonal code symbol in an orthogonal code to a secondary station irrespective of the channel activity during a call set up, and the secondary station returns the orthogonal code symbol to the primary station for another secondary station after termination of the call.
In the description of the prior art, the same reference number will be assigned to the parts having the same function as in the description of the present invention.
FIG. 1 is a schematic of a system according to an example of the prior art and an embodiment of the present invention, in which channels 121, 122 and 123 formed from a primary station 101 to secondary stations 111, 112 and 113 are in synchronization with one another and have mutual orthogonality.
FIG. 2a is a schematic of a transmitter of the primary station for a part corresponding to the common component between the prior art and the present invention, and FIG. 2b is a schematic of a transmitter of the primary station for a traffic channel in the example of the prior art. A pilot channel 200 must be present by the respective subcarriers SCs, because it is used as a channel estimation signal for initial synchronization acquisition and search and synchronous demodulation at the secondary stations of FIG. 1. The pilot channel 200 is a channel shared among all secondary stations in an area that is under the control of the primary station. As illustrated in FIG. 2a, the pilot channel 200 is used to provide a phase reference for synchronous demodulation by transmitting a symbol of a known pattern without channel coding or channel interleaving. Like the pilot channel 200, a synchronous channel 210 is a broadcasting channel uni-directionally transferred to all the secondary stations in an area that is under the control of the primary station. The synchronous channel 210 is used for the primary station to transfer information (e.g., visual information, the identifier of the primary station, etc.) required in common to all the secondary stations. The data through the synchronous channel are sent to a spreader and modulator, which will be described later in FIG. 3, via a convolutional encoder 214, a repeater 216 for symbol rate control, a block interleaver 218 to overcome burst errors, and a repeater 219 to control a transmit data symbol rate. A paging channel 220 is a common channel used in the presence of an incoming message to the secondary station or for the purpose of responding to the request of the secondary station. Plural paging channels can be used.
The data transmitted through the paging channel are sent to an exclusive OR operator 236 via a convolutional encoder 224, a symbol repeater 226 and a block interleaver 228. The output of a long code generator 232 is sent to a decimator 234, which decimates the output of the long code generator 232 using a long code mask for paging channel 230. The exclusive OR operator 236 exclusive-OR operates the data from the block interleaver 248 with the decimated output of the long code generator 232 and then sent to the spreader and modulator of FIG. 3. A traffic channel 240 of FIG. 2b is a channel allocated to each secondary station during a call set up and exclusively used by the secondary station until a call termination. The traffic channel is used to transfer data from the primary station to each secondary station. The traffic channel is sent to a CRC (Cyclic Redundancy Check) encoder 241 to check errors in the unit of a predetermined time called a frame (e.g., 20 ms in the IS-95 (cdmaOne) system), a tail bit inserter 252 to insert tail bits that are all “0” for independent channel coding in the unit of frames, a convolutional encoder 244 and then a symbol repeater 246 to correct the transmit data symbol rate according to the transmit data rate.
Subsequent to symbol repetition, the traffic channel is sent to a block interleaver 248 to convert burst errors to uniformly distributed errors, and then to a scrambler 256. The output of the long code generator 232 is decimated into a PN (Pseudo-Noise) sequence by the decimator 234 using the long code mask 250 generated from an ESN (Electronic Serial Number) allocated by the respective secondary stations. The scrambler 256 scrambles the traffic channel from the block interleaver 248 using the PN sequence.
The scrambled traffic channel is sent to a PCB (Power Control Bit) position extractor 258 to extract a PCB position from the PN sequence to insert a PCB for controlling the transmission power from the secondary station. A PCB puncture and insert section 260 punctures a data symbol corresponding to the PCB position among the scrambled data symbols from the scrambler 256, and inserts a PCB. The PCB-inserted traffic channel is sent to the spreader and modulator of FIG. 3.
The position of the transmit data symbol for transmission time hopping multiplexing according to the present invention can also be detected using the decimated PN sequence as described above.
FIGS. 3a, 3b and 3c illustrate an example of the spreader and modulator using the conventional code division multiplexing technology.
The spreader and modulator of FIG. 3a uses the existing IS-95 (cdmaOne) system based on a BPSK (Binary Phase Shift Keying) data modulation system.
The spreader and modulator of FIG. 3b spreads I/Q channel transmit data with a different orthogonal code symbol in the structure of FIG. 3a. The spreader and modulator of FIG. 3c employs a QPSK (Quadrature Phase Shift Keying) data modulation system so as to transmit double the data of FIG. 3a with the same bandwidth. The QPSK data modulation system is adapted to cdma2000, one of the candidate technologies of the IMT-2000 system.
The spreader and modulator of FIG. 3d use the QPSK data modulation system in order to transmit double the data of FIG. 3b with the same bandwidth. FIG. 3e shows a spreader and modulator using a QOC (Quasi-Orthogonal Code) modulation system usually adapted in cdma2000, one of the candidate technologies of the IMT-2000 system.
FIG. 3f shows that I/Q channel transmit data are spread with a different orthogonal code symbol in the structure of FIG. 3e. 
Referring to FIG. 3a, signal converters 310, 330, 326, 346 and 364 convert logic signals of “0” and “1” to actual transmit physical signals of “+1” and “−1”, respectively. The individual channels of FIG. 2 are sent to spreaders 312 and 332 via the signal converters and spread with the output of a corresponding Walsh code generator 362. The spread channels are then sent to amplifiers 314 and 334 to control their relative transmission power.
After passing through the spreaders 312 and 332 using an orthogonal Walsh function 362 fixedly allocated to each channel and the amplifiers 314 and 334, the channels of the primary station are all sent to orthogonal code division multiplexers 316 and 336.
The multiplexed signals are then sent to QPSK spreader and modulators 318 and 338 using short PN sequences generated from short PN sequence generators 324 and 344 for discrimination of the primary station. The spread and modulated signals are sent to low-pass filters 320 and 340 and modulators 322 and 342 for transition to a transmit band. The signals modulated with carriers are sent to a wireless section (not shown) such as a high power amplifier and then transferred via an antenna.
Referring to FIG. 3b, signal converters 310, 330, 326, 346 and 364 convert logic signals of “0” and “1” to actual transmit physical signals of “+1” and “−1”, respectively. The individual channels of FIG. 2 are sent to spreaders 312 and 332 via the signal converters and spread with the output of a corresponding Walsh code generator 362 by the I/Q channels. The spread channels are then sent to amplifiers 314 and 334 to control their relative transmission power. After passing through the spreaders 312 and 332 using an orthogonal Walsh function 362 fixedly allocated to each channel and the amplifiers 314 and 334, all the channels of the primary station are sent to orthogonal code division multiplexers 316 and 336. The multiplexed signals are then sent to QPSK spreader and modulators 318 and 338 using short PN sequences generated from short PN sequence generators 324 and 344 for discrimination of the primary station. The spread and modulated signals are sent to low-pass filters 320 and 340 and modulators 322 and 342 using carriers for transition to a transmit band. The signals modulated with carriers are sent to a wireless section (not shown) such as a high power amplifier and then transferred via an antenna.
FIG. 3c is the same as FIG. 3a, excepting that the signals generated in FIG. 2 are sent to a demultiplexer 390 for QPSK, rather than BPSK, using an in-phase (I) channel and a quadrature phase (Q) channel in sending different information data. The demultiplexer 390 and the signal converters 310 and 330 are used to realize QAM (Quadrature Amplitude Modulation) instead of QPSK.
FIG. 3d is the same as FIG. 3b, excepting that the signals generated in FIG. 2 are sent to a demultiplexer 390 for QPSK, rather than BPSK, using an in-phase (I) channel and a quadrature phase (Q) channel in sending different information data.
FIG. 3e shows that the transmit data are spread with a spreading code generated using the quasi-orthogonal code mask for discrimination of a channel from the primary station to the secondary station in FIG. 3c. Orthogonality is not maintained in the code symbol group using a different quasi-orthogonal code but in the code symbol group using a same orthogonal code mask.
Accordingly, the system proposed in the present invention is applied only to the orthogonal code symbol group using a same quasi-orthogonal code mask and maintaining orthogonality.
FIG. 3f is the same as FIG. 3e, excepting that a separate Walsh code generator is used for I- and Q-channels so as to spread I/Q channel transmit data with a different orthogonal code symbol.
FIGS. 4b and 4c show a signal diagram explaining a multiplexing method in which orthogonal resources are allocated to the signals generated in FIGS. 2 and 3 by the respective channels to transmit the signals.
With a primary station in communication with secondary stations, the data rate by the respective secondary stations may be variable over time. Let the channel-based maximum transmission rate allocated to the secondary stations by the primary station be a basic transmission rate R, the frame-based average transmission rate may be R, R/2, R/4, . . . , or 0 according to the frame-based amount of data transferred from the primary station to the secondary stations.
FIG. 4b is a signal diagram showing that the frame-based instantaneous transmission rate is adjusted to the average transmission rate, which method is applied on the forward link in the IS-95 (cdmaOne) orthogonal code division multiplexing communication systems.
In FIG. 4b, when the frame-base transmit data have a transmission rate below the basic transmission rate, dummy information is used to compensate for the deficient part and thereby match the frame-based instantaneous transmission rate to the average transmission rate.
FIG. 4c shows that the instantaneous transmission rate is classified into a basic transmission rate R and 0 (no transmission) and that an average transmission rate for a given frame is adjusted according to the percentage of an interval having a transmission rate of R or 0.
In FIG. 4c, instead of the ON/OFF switching of transmit symbol units that are spreading units used in the present invention, the ON/OFF switching of time slot units that are power control units is used in adjusting the frame-based average transmission rate, while maintaining the amplitude of the reference signal for a closed loop power control of the reverse link in the IS-95 (cdmaOne) system.
Contrary to the present invention, there is no orthogonality between channels on the IS-95 (cdmaOne) reverse link.
In FIGS. 4b and 4c, the common pilot channel is used in parallel with a channel to the secondary stations. But the pilot channel, which is used at the receiver as a reference for synchronization, channel estimation and power control, can be transmitted by time division multiplexing as in the conventional GSM (Global System for Mobile) or WCDMA (Wideband CDMA) system. The pilot channel in this case is called “pilot symbol” or another various names such as preamble, midamble or post-amble according to the multiplexed position.
FIG. 4d illustrates the conventional frequency division multiplexing system, in which communication channels from a primary station to plural secondary stations use a different frequency allocation (FA). The frequency division multiplexing system of the present invention includes the OFDM (Orthogonal Frequency Division Multiplexing) system actively studied for satellite broadcasting.
For OFDM, the FA of the individual subcarrier channels is not completely independent but overlapped, but may be included in the orthogonal resource of the present invention, because orthogonality between the subcarrier channels is secured.
FIG. 4e illustrates the conventional time division multiplexing system such GSM, in which communication channels from a primary station to plural secondary stations use a same frequency allocation (FA) but the time slots in the frame are exclusively allocated by the respective secondary stations.
FIGS. 4f, 4g and 4h apply a frequency hopping system to the conventional frequency division multiplexing system of FIG. 4d for the purpose of strengthening frequency diversity and security. FIG. 4f shows frequency hopping in the unit of time slots. FIG. 4g shows regular frequency hopping in the unit of transmit data symbols. FIG. 4h shows irregular frequency hopping in the unit of transmit data symbols. The system of FIG. 4g brings focus into frequency diversity, and that of FIG. 4h has importance on frequency diversity and security for preventing a monitoring by an unauthorized receiver.
Frequency hopping multiplexing includes fast frequency hopping multiplexing in the unit of symbols or partial symbols and slow frequency hopping multiplexing in the unit of several symbols. The systems of FIGS. 4f, 4g and 4h applied to the time division multiplexing system of FIG. 4e provide frequency diversity.
In fact, the use of frequency hopping in the unit of time slots or frames is optionally given in the next-generation mobile communication system, i.e., GSM for the purpose of strengthening frequency diversity rather than security.
FIG. 4i illustrates the conventional orthogonal code division multiplexing system such as cdma2000 or WCDMA systems. In FIG. 4i, the communication channels from a primary station to the respective secondary stations are established using the same frequency allocation (FA) and all time slots in the frame. The primary station allocates a fixed orthogonal code symbol to each channel during a call set up, and each secondary station returns the orthogonal code symbol to the primary station for another secondary station involving another call set up. Accordingly, all the data symbols in the frame are spread with the same orthogonal code symbol. The transmitters of the primary station corresponding to FIG. 4i are presented in FIGS. 3a to 3f. 
FIG. 4j is a signal diagram of a transmit signal from the primary station in the conventional ORDM (Orthogonal Resource Division Multiplexing) system, in which channel-based fixed allocation of orthogonal resources is illustrated. ORDM is applied to most of the conventional digital communication systems.
The receiver of the secondary station corresponding to the transmitter of the primary station according to the example of the prior art as shown in FIG. 4i is similar to the transmitters of FIGS. 3a to 3f except for the despreading part. Thus, FIG. 5 shows a schematic view of a receiver corresponding to the transmitter of FIG. 3a. The received signal through an antenna is demodulated with carriers at demodulators 510 and 530 and low-pass filtered at low-pass filters 512 and 533 into a baseband signal. The sequences, which are generated from PN-I/Q short code generators 520 and 540 and the same with PN sequences used at the transmitter, are synchronized and multiplied by the received baseband signal at multipliers 514 and 534. The multiplied sequences are cumulated for a transmit data symbol interval and sent to despreaders 516 and 536. A channel estimator 550 extracts a pilot channel component from the baseband signal with an orthogonal code symbol allocated to the pilot channel to estimate a transmit channel, and a phase recovery section 560 compensates for the phase distortion of the baseband signal using the estimated phase distortion value. If the pilot channel is subject to time division multiplexing rather than code division multiplexing, a demultiplexer is used to extract the pilot signal part and the intermittent phase change between the extracted pilot signals is then estimated by interpolation.
FIG. 6 shows the structure of a receiver for a channel without a PCB insertion from the primary station like the above-stated paging channel, where the PCB is a command for controlling the transmission power from the secondary stations to the primary station. After the phase compensation in FIG. 5, the signals are fed into maximum ratio combiners 610 and 620. With a QPSK data modulation at the transmitter as illustrated in FIG. 3b, the combined signal is sent to a multiplexer 614 for multiplexing; alternatively, with a BPSK data modulation, the two signals are added. The resulting signal is then sent to a soft decision section 616 for soft decision. The output of a long code generator 622 formed by a long code mask 620 is sent to a decimator 624. The signal from the soft decision section 616 is multiplied by the decimated output of the long code generator 622 by a multiplier 618 for descrambling. The receiver of the secondary station for a channel subject to orthogonal code hopping multiplexing according to the embodiment of the present invention is similar in structure to the receiver of FIG. 6. For the synchronous channels, the components related to the long code descrambling process are omitted.
FIG. 7 shows the structure of a receiver for a channel with a PCB insertion from the primary station like the above-stated traffic channel, where the PCB is a command for controlling the transmission power from the secondary stations to the primary station. After the phase compensation in FIG. 5, the signals are fed into maximum ratio combiners 710 and 720. With a QPSK data modulation at the transmitter as illustrated in FIG. 3c, the in-phase (I) component and the quadrature (Q) phase component are sent to a multiplexer 714 for multiplexing; alternatively, with a BPSK data modulation as illustrated in FIG. 3a, the in-phase (I) component and the quadrature phase (Q) component are added. The resulting signal is sent to an extractor 740 for extraction of a signal component corresponding to the PCB from the primary station and then to a hard decision section 744 for hard decision. The signal from the hard decision section 744 is transferred to the transmission power controller of the secondary station. The data symbol generated by removing the PCB from the received signal of the multiplexer 714 is sent to a soft decision section 742 for soft decision. The output of a long code generator 722 formed by a long code mask 720 generated from the identifier of the secondary station is sent to a decimator 724. The signal from the soft decision section 742 is multiplied by the decimated output of the long code generator 722 by a multiplier 718 for descrambling.
FIG. 8 illustrates that the received signal processed in FIG. 7 is subject to channel deinterleaving at block deinterleavers 818, 828 and 838 and channel decoding at convolutional decoders 814, 824 and 834 to reconstitute data transferred from the primary station. For a synchronous channel 810, the signal from the soft decision section is sent to a sampler 819 for symbol compression that is a reversed process of the symbol repeater 219 by accumulation of the received signals, thereby reducing a symbol rate. The signal from the sampler 819 is sent to the block deinterleaver 818 for channel deinterleaving. Before a channel decoding at the convolutional decoder 814, the channel-deinterleaved signal is sent to a sampler 816 for another symbol compression that is a reversed process of the symbol repeater 216. The signal from the sampler 816 is sent to the convolutional decoder 814 for channel decoding, thereby reconstituting the synchronous channel received from the primary station. For a paging channel 820, the signal from the soft decision section is sent to the block deinterleaver 828 for channel deinterleaving. The channel-deinterleaved signal is sent to a sampler 826 for symbol compression according to the transmit data rate that is a reversed process of the symbol repeater 226. The signal from the sampler 826 is sent to the convolutional decoder 824 for channel decoding, thereby reconstituting the paging channel received from the primary station. For a traffic channel 830, the signal from the soft decision section is sent to the block deinterleaver 838 for channel deinterleaving irrespective of the transmit data rate. The channel-deinterleaved signal is sent to a sampler 836 for symbol compression according to the transmit data rate that is a reversed process of the symbol repeater 246. The signal from the sampler 836 is sent to the convolutional decoder 834 for channel decoding and removed of a tail bit for frame-based independent transmit signal generation by a tail bit remover 832. As in the transmitter for the transmit data part, a CRC bit is generated and compared with a CRC bit reconstituted by channel decoding to determine whether or not there is an error. When the two CRC bits are matched, it is determined that there is no error, thereby reconstituting traffic channel data. If information about the transmit data rate in the unit of 20-ms frames is not stored at the transmitter, the channel deinterleaved signals are channel-decoded independently for all possible transmit data rates and then the CRC bits are compared to determined the transmit data rate from the primary station. For a system in which the transmit data rate is separately transferred, only the channel decoding process for a corresponding data rate is needed.
There are four conventional methods for maintaining orthogonality between channels from the primary station to the secondary stations as shown in FIG. 1. The first method is to use the frequency division multiplexing so that the primary station fixedly allocates FA to the secondary stations during a call set up as illustrated in FIG. 4d. The second method is to use the time division multiplexing so that the primary station fixedly allocates time slots to the secondary stations during a call set up as illustrated in FIG. 4e. The third method is to allocate a hopping pattern controlled to avoid a collision of the primary station during a call set up as illustrated in FIGS. 4f, 4g and 4h to the secondary stations, or to use the total bandwidth composed of multiple subcarriers for a single secondary station at a given time in a given area, as in the military use. The fourth method is to allocate unoccupied orthogonal code symbols by the primary station during a call set up and spreading channels as illustrated in FIG. 4i. 
Apart from the frequency hopping multiplexing method, the other three methods have a common feature that the primary station allocates fixedly orthogonal resources (e.g., frequency, time, or orthogonal code) to the secondary stations. The frequency hopping multiplexing method is primarily used for the security purpose in many applications supporting a sufficient quantity of resources, as in the military use. Hence, the frequency hopping multiplexing is not aimed at an efficient use of resources.
It is therefore difficult in the above methods to efficiently use the resources when the limited orthogonal resource is allocated to the channels having a low activity or a variable transmit data rate less than or equal to a basic transmission rate.
A rapid channel allocation and release is required in order to fixedly allocate the resources as in the prior methods and increase the utilization of the resources. But, a considerable part of the confined resources are not used for actual data transmission but allocated to the control information for data transmission because the control signal information for frequent channel allocation and release is transferred.
Even with a rapid procedure of channel allocation and release, the data to be transmitted must be buffered during a period from its arrival at the primary station to transmission via the steps of channel allocation (or release) message transmission and confirmation. The required buffer capacity in this case increases with an increase in the processing time of the procedures.
In the method of fixedly allocating resources during a handoff to an adjacent cell, the handoff is hardly acquired even when the channels in the adjacent cell have a low activity, because there is no available resource to be allocated.
Furthermore, important information such as control information that necessarily requires a confirmation step after a transmission must be buffered for retransmission. But the required buffer capacity can be reduced only by transmitting the resources with a shortest delay in the transmission such as datagram transmission that does not require a confirmation step.